Energy storage, power generation, and power quality devices often require electronic converters, switches, and various controls to interface with the power grid, the load, and each other. This is particularly true when one or more such devices are used together. Combining one or more such devices often entails expensive duplication of electronics, switches, and control. Combining devices also often requires oversizing equipment and substantial wiring between components, and raises issues of equipment incompatibility and non-ideal overall system performance.
The present invention addresses the problem of efficiently combining a variety of storage and generation devices, for the purpose of providing high quality power and reliability to a load and interfacing of the storage and generation devices to the power grid for purposes of energy control, load leveling and peak shaving. The following sections discuss power quality/reliability devices, generators and storage devices, with emphasis on aspects of these devices that relate to interfacing them with each other, the power grid, and various loads.
Power Quality and Reliability
Power quality and reliability is usually obtained through some form of uninterruptible power supply (UPS). Standard UPSs provide continuous power to a load, even when the power grid is interrupted. In the event of a power grid interruption, a UPS will provide conditioned power to a load through the use of energy storage, typically batteries, for a short period of time ranging from milliseconds (in the case of small capacitor storage) to minutes (in the case of batteries). To obtain longer ride-through in the event of a power grid interruption, a reliable form of generated power is used.
A standard method for obtaining long term uninterrupted power involves connecting a standard UPS to an automatic transfer switch that can be powered either from the power grid or a backup generator. Such a configuration requires a substantial amount of wiring between all of the components and often requires the user to deal with communication and compatibility issues. Additionally, standard on-line UPS systems do not present a clean load to the generator, thus requiring oversizing of the generator, extra filtering, or both.
Systems that have a backup generator have the potential to operate the generator for energy savings with the addition of other system components. Additional components may include but are not limited to the following: system protection components, mechanisms for synchronizing with the grid, controls to ensure that the generator turns on and produces an appropriate amount of power at the appropriate time, safeguards to prevent power flow into the power grid, appropriate metering when reverse power flow is allowed, and communication with a system that provides real-time pricing of electricity. Such systems might also require additional communication if the generator is to be turned on remotely for purposes of safety, convenience, or possibly as part of a large system composed of many smaller generators (effectively a distributed utility).
Energy Storage Devices
a. Flywheel Energy Storage
Flywheels can provide a mechanism of energy storage for an electrical system. Mechanical energy is transformed into electrical energy (and vice-versa) through a motor-generator. To provide a reasonable amount of energy storage, flywheels operate over a large speed range. The power electronics that connect the flywheel to the electrical system must therefore operate over a wide band of frequencies, and sometimes over a large range of voltages, depending on whether the flywheel motor-generator has an adjustable field coil.
Because flywheels operate over a wide band of frequencies, they cannot interface directly with the power grid or a load. An AC-to-AC converter is used to interface the flywheel with the power grid or the load. The AC-to-AC converter can either convert the AC flywheel voltage directly to AC voltage at the power grid frequency (and similarly in the reverse direction), or the converter can first rectify the AC voltage of one, and then invert the resulting DC voltage into the AC voltage with the appropriate amplitude and frequency of the other.
b. Battery Storage
Batteries are a standard method of storage for UPS systems. They operate at nearly constant DC voltage and so require an inverter to interface with the power grid or with an AC load. A controlled rectifier is used to charge the battery from the power grid or some other AC supply, such as from a generator.
c. Capacitor Storage
A small amount of capacitor storage can be obtained by placing capacitors, such as electrolytic capacitors, onto a DC bus. Super capacitors can also be used for this purpose, if they have a high enough peak output current for the application.
The useful amount of energy storage in a capacitor can be increased significantly by placing a DC-to-DC converter at the output. Thus, the output voltage can be held constant over a large range of capacitor charge. The DC-to-DC converter adds a significant amount of cost to the storage system.
d. Other Forms of Energy Storage
Other forms of energy storage exist, such as compressed air storage and super conducting magnetic energy storage. These other forms of energy can usually be interfaced with a DC bus through the addition of a controlled or uncontrolled rectifier. They sometimes also require a DC-to-DC converter.
Power Generation
a. Low Speed Synchronous Generators
The majority of generators in existence are low speed synchronous generators. The output of these generators is a nearly sinusoidal voltage at the same frequency as the power grid. These generators can therefore be connected directly to an AC load.
b. Low Speed Induction Generators
Low speed induction generators must spin slightly faster than corresponding synchronous generators to generate any power. Furthermore, an external AC voltage source is placed in parallel with an induction generator for any power generation to occur. Thus, an induction generator could generate power directly into the grid, but it could not generate power directly into a load without the presence of another voltage source such as might be provided by an AC inverter.
Low speed induction generators exist primarily for applications which allow for grid-parallel operation--that is, the generator is placed in parallel with the grid, and the prime mover operates at a speed slightly faster than synchronous speed. Prime movers for induction generators do not have to operate at constant speed. They are thus useful for wind and water turbines. They can also be used with gas or steam powered turbines which were designed primarily for grid-parallel operation. Other advantages of induction generators include easy starting, low cost, and ruggedness.
c. High Speed Permanent Magnet Generators
In the past, most generators have been designed to produce power at the power grid frequency. Recent advances in materials and power electronic components has led to the production of high speed generators. The prime mover of the high speed generators can be a small turbine or anything that operates significantly faster than 3600 rpm for 60 Hz or 3000 rpm for 50 Hz supplies. High speed generators cannot be connected directly to most loads or the power grid because they operate at a frequency different from, and much higher than, the power grid frequency. High speed generators therefore require power electronics to interface with a load or to the power grid. The electronics required to interface a high speed generator with a load are similar to the electronics required for a UPS.
Recent improvements in permanent magnet and other materials has led to the production of high speed permanent magnet generators, especially for use with microturbines. When connected to a micro-turbine, these generators are typically operated as motors for starting. During this starting phase, an inverter supplies high frequency AC voltage to the stator. After the turbine has come up to speed, it provides power to a load, or directly to the power grid, through a power electronics converter. This converter often consists of a rectifier and a DC-to-AC inverter.
d. High Speed Induction Generator
High speed induction generators can be used with the same prime movers as high speed permanent magnet generators. High speed induction generators require an inverter to produce any power. This power must then be transformed to a lower frequency through the use of some form of AC-to-AC converter. The starting inverter for a high speed induction generator would be similar to that for a permanent magnet generator with the exception of simpler control and no need for very low frequency operation.
e. Other Forms of Generation
Other forms of generation exist such as fuel cells and solar cells. These forms of generation usually produce variable DC, and thus must use a DC-to-AC inverter to interface with the power grid or an AC load. In some cases, a DC-to-DC converter located between the DC bus of the inverter and the generation device reduces the size and cost of the inverter.